docs: def _validate_indices(self, indices=None):
          """Return a valid list of indices.
  
          `indices` may be a string or a list of strings.
          If `indices` is not supplied, returns the default_indices.
          """
          if indices is None:
              indices = self.default_indices
          if isinstance(indices, six.string_types):
              indices = [indices]
          return indices